Many types of electronic apparatus are known in which the various electrical components are interconnected by electrical conductors. The interconnecting conductors are fabricated in a wide variety of processes such as, for example, thick-film fired conductor systems, polymer conductors and printed circuit boards.
In thick-film fired conductors, a mixture of a conducting metal powder, a ceramic or glass binder and an appropriate vehicle is screen printed on a substrate. The conductor pattern on the substrate is then fired at a relatively high temperature, typically between 650.degree. and 900.degree. C. As the temperature rises to the firing temperature, the vehicle is volatilized leaving the metal and binder behind. At the firing temperature, sintering of the metal takes place to a greater or lesser extent with the binder providing adhesion between the metal film formed and the substrate.
Thick-film fired conductors have classically employed precious metals such as gold, silver, platinum and palladium. Recently three noble metals have soared in cost, and new conductor systems using copper, nickel and aluminum are being made commercially available. The cost of the precious metal systems is prohibitive where a low cost conductor system is desired. The newer metal systems are not significantly cheaper because of the special chemistry which is required to prevent oxidation of the metal during the firing process. Moreover, these systems are very difficult to solder using the conventional tin/lead solder and the high firing temperatures required during fabrication preclude the use of low cost substrate materials. Some of the nickel system can be fired on soda-lime glass at temperatures just below the melting point of the glass but the resulting conductivity of the conductor is relatively low.
The term "polymer conductor" is actually a misnomer since the polymer is not actually a conductor. Instead, the polymer is heavily loaded with a conducting metal and screened on to a substrate. The advantage of this system is that the polymer can be cured either catalytically or thermally at temperatures which range from room temperature to about 125.degree. C. As a result of this so called "cold processing", it is possible to use very inexpensive substrates such as films of MYLAR.RTM.(polyethylene terephthalate). The mechanism by which conductivity is achieved is supplied entirely by contact between individual metallic particles. It has been found that the only metals which can be loaded into the polymer and give acceptable conductivity are the precious metals such as gold and silver. All of the other standard conducting metals oxidize over a period of time and the conductivity between the particles is reduced. Silver has been the predominant choice in polymer conductor systems but the silver systems are generally not solderable because the silver is leached by the lead-tin solder. When the price of silver is about $10-11 per ounce, these conductor systems are competitive with other systems if used on very low cost substrates such as thin MYLAR films. However, when the price of silver is higher, the systems are not competitive with printed circuit boards.
The techniques used to prepare printed circuit boards can be divided into additive and subtractive technologies. In both, the starting point is a substrate, which can vary widely from phenolics to glass-filled epoxies, on which a copper foil is bonded. In the additive preparatory system, the copper foil is very thin, usually on the order of about 200 microinches. A resist is patterned such that the copper is exposed only where the conductors are desired and the board is then electroplated to form copper conductors of about 1 mil in thickness. The plating resist is stripped and the copper is etched. In areas where the conductor is not desired, the copper is only about 200 microinches thick so that etching quickly removes this copper while leaving a 1 mil thick conductor. In the subtractive process, the starting thickness of the copper foil is usually between 1 and 2 mils. An etch resist is deposited wherever the conductors are desired, the board is etched and the resist is then removed. The resist prevents etching where the conductors are desired leaving conductor runs.
Both the additive and subtractive printed circuit board procedures require the application of a copper foil over the entire substrate, deposition and removal of a resist, etching of the printed circuit board, drilling holes for component insertion, and in one case, the additional step of electroplating. An advantage of this technology is, however, that the resulting circuit boards can be relatively easily soldered.
Another advantage of the printed circuit board technology is that plated-through holes can be fabricated. This process usually involves the addition of several steps to the additive fabrication process. Holes are drilled in the substrate and thereafter the resist is applied over all areas except where the conductors are desired. The board is then soaked in a stannous chloride sensitizer which forms a coating over the exposed parts of the substrate, namely inside the holes. The board is then sequentially dipped in a bath of palladium chloride, acid to dissolve the tin chloride, and an electroless copper bath. The last step, i.e., immersion in an electroless copper bath, deposits a very thin film of copper inside the activated hole. This "electroless copper" is plated out by a catalytic reaction in which the catalyst is copper such that a continuous plating reaction can occur. Typically, thickness on the order of 24-50 microinches can be achieved in 0.5 hour. At this point, a thin film of copper is adhered to the inside edges of the holes. The subsequent electroplating will build up the thickness of the copper within the holes as well as along all of the conductor runs. At this point, the various processes employed differ. The simplest process merely strips the resist and then etches, eliminating the much thinner copper where the conductor runs are not desired. In more complex processes, electroplating of tin-lead solder is accomplished which results in a tin-lead solder inside the hole and along the conductor runs, followed by stripping the plating resist and etching with chromic acid, which does not attack the tin-lead solder so that the solder acts as an etch resist.
The most significant drawbacks of the printed circuit board technology is that a substantial number of processing steps are necessary and this requires a large amount of associated equipment. In addition, the choice of substrate materials is limited to one of those available for circuit board materials. The number of process steps and equipment results in relatively high processing costs and the limitation of the substrate materials eliminates the opportunity to use a decorative or structural member, which may already be required in the apparatus, as the substrate. Typical total costs for processed printed circuit boards range from $0.03 to $0.15 per square inch depending on the quality of the board, whether the board is single-sided or double-sided and whether plated-through holes are used.
When polymer resistors are printed directly on thick-film substrates or printed circuit boards, resistor termination problems occur where a junction to a thick film printed resistor is provided by overlapping the underlying conductor with the resistor pattern. The area of overlap provides the connection between the conductive pathway and the resistor, and if the underlying conductor is oxidized or the resistor material is incompatible with the underlying conductor, a poor termination is formed. This problem is especially prevalent when the resistors have low resistivity because the resistance of the interface can provide a substantial proportion of the total resistance. A further problem exists when it is desirable to cure the resistor at relatively high temperatures, such as 200.degree. C. Under these conditions, the conductor surface is oxidized, making it considerably more difficult to obtain good solder wetting to the conductor during subsequent soldering cycles.
In thick-film circuits, the conventional method of attaching components to conductor runs of a circuit is by soldering. This requires that the substrate be dipped in a hot solder bath or that a solder/rosin paste be printed in the area where the solder is desired. The components to be connected are mounted on the surface of the substrate and the resulting structure is heated to the melting point of the solder to reflow the solder so as to connect the components to the conductors. This method, however, engenders several problems. A number of process steps are required to establish the solder in place before the part can be mounted and a reflow soldering step must still be undertaken. Also, problems result from the requirement that the temperature be above the melting point of the solder. In some systems, especially those using a plastic substrate, soldering temperatures on the order of 230.degree. C. cannot be accommodated. Still another problem stems from the additional processing steps which are required to make component leads compatable with the soldering process, i.e., the leads must either be solder or gold plated. A typically used lead material is the alloy KOVAR.RTM., which has a relatively ideal coefficient of expansion but which is not solderable itself. In the case of solder dipping followed by reflow, it is necessary to use some form of jig to hold the components in place because the solder is hard and the components will not stick to its surface. Solder pastes overcome this problem somewhat but they are much more expensive. A further problem with the use of a solder system occurs when the coefficient of expansion of the component does not closely match that of the substrate; under these conditions, the solder connection can be broken as a result of the thermal stresses developed.